Remove unused imports in pub.

sdk/lib/_internal/pub/lib/src/package.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.package;
 
-import 'dart:async';
-
 import 'package:path/path.dart' as path;
 
 import 'io.dart';
 import 'pubspec.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
 
 final _README_REGEXP = new RegExp(r"^README($|\.)", caseSensitive: false);
 
@@ skipping 194 matching lines @@
 class PubspecNameMismatchException extends ApplicationException {
   final String expectedName;
   final String actualName;
 
   PubspecNameMismatchException(String expectedName, String actualName)
       : expectedName = expectedName,
         actualName = actualName,
         super('The name you specified for your dependency, "$expectedName", '
               'doesn\'t match the name "$actualName" in its pubspec.');
 }